The present invention relates to a magnetic tape cassette, and more particularly to a magnetic tape reel used in a magnetic tape cassette for use in a VTR or the like.
Although they are classified into various types such as the type and VHS type, etc., magnetic tape cassettes for use in VTRs (video tape recorders) are generally similar to each other in basic structure, namely, the magnetic tape is housed in a cassette wound around a pair of magnetic tape reels. As shown in FIGS. 4 and 5, the reel structure includes a tape winding drum portion 2 provided with a plurality of concentric cylindrical portions including an inner cylindrical portion 6 engaged with a rotary driving shaft 30, an outer cylindrical portion 3 disposed outside the inner cylindrical portion 6 so as to form a tape winding surface 3, and, if necessary, an intermediate cylindrical portion disposed between the outer cylindrical portion 3 and the inner cylindrical portion 6. Also, a plurality of reinforcing ribs 15 are provided extending in the radial directions of the reel between the inner cylindrical portion 6 and the outer cylindrical portion 3, and flanges 8 and 9 are provided on the tape winding drum portion 2 at its lower and upper sides for causing the magnetic tape to be wound evenly and neatly. The reinforcing ribs 15 are provided in the radial directions of the reel for the purposes of improving the mechanical strength of the tape winding drum portion 2 and improving the resin flow when a reel 20 is injection molded with molten plastic resin.
Conventionally, there has been a problem in that portions of the reinforcing ribs 15 shrink in the radial directions of the reel because of the unavoidable shrinkage of the resin just after molding, whereby that the roundness or cylindricity of the tape winding drum portion 2 is disturbed.
In order to eliminate this problem, various proposals have been made. For example, in Japanese Examined Utility Model Publication No. 57-28313, a magnetic tape winding core is constituted by an inner cylindrical body having reinforcing ribs connected to the inner circumferential surface thereof, and an outer cylindrical body provided around the inner cylindrical body with a gap therebetween. In the configuration disclosed in the Japanese Utility Model Publication No. 57-28313, however, the problem remains in the case of a resin such as polyoxymethylene (POM) or the like which is widely used for molding reels, because the resin has a large thermal shrinkage characteristic. The configuration is generally effective though in the case of a resin such as ABS resin which undergoes relatively little shrinkage due to the reduction in temperature following injection molding.
In order to solve the this problem, the inventors have proposed a further improved reel structure in Japanese Unexamined Utility Model Publication No. 61-79886. According to this structure, the roundness and cylindricity of the tape winding drum portion have been remarkably improved.
However, the inventors have found a new problem through still further investigation and development. The problem is that, depending on the conditions of use and storage of a cassette, the reinforcing ribs 15 are sometimes urged in the inward radial directions of the reel by the winding and fastening pressure of the magnetic tape wound around the tape winding drum portion 2. Thus, a transformation sometimes occurs which reduces the diameter of hub holes 7 engaged with the rotary driving shaft 30 of the recording/reproducing apparatus. If such a transformation occurs while the cassette is in storage, it may become impossible for the hub hole 7 to engage with the rotary driving shaft 30. Further, if such a transformation occurs when the cassette is loaded in a recording/reproducing apparatus, it may become impossible to remove the cassette without damage.